The present invention relates to improving the low load operation of fuel burners for use in pulverized coal-fired furnaces and, more particularly, to improving low load operation of fuel-air admission assemblies for directing a pulverized fuel-air mixture into the furnace by what is known as the tangential method of firing.
In view of today's fluctuating electricity demand, typified by peak demand occurring during weekday daytime hours and minimum demand occurring at night and on the weekends, electric utilities have chosen to cycle many of their conventional coal-fired steam generator boilers by operating them at full load during peak demand hours and reducing them to low loads during periods of minimum demand.
As a consequence of this mode of operation, the electric utilities have used large quantitites of natural gas or oil to furnish additional ignition energy during low load operation because the current generation from coal-fired steam generator furnaces require stabilization of the coal flames when operating at low loads. The required amount of auxiliary fuel fired for stabilization purposes is significant and, for example, to maintain a 500 megawatt coal-fired steam generator at 10 to 15 percent load during minimum demand periods could require the use of 11,000 gallons of oil per day.
One common method of firing a pulverized fuel such as coal in a conventional steam generator furnace is known as tangential firing. In this method, pulverized coal is introduced to the furnace in a primary air stream through burners, termed fuel-air admission assemblied, located in the corners of the furnace. The fuel-air streams discharged from these assemblies are aimed tangentially to an imaginary circle in the middle of the furnace. This creates a fireball which serves as a continuous source of ignition for the incoming coal. Each fuel-air admission assembly is comprised of a fuel delivery pipe through which pulverized fuel entrained in air passes to the furnace, a secondary air conduit surrounding the fuel delivery pipe through which additional air is introduced into the furnace, and a nozzle tip which is pivotally mounted to the outlet end of the fuel delivery pipe.
A typical nozzle tip comprises inner and outer shells disposed coaxially in spaced relationship thereby defining a first flow passageway within the inner shell through which the pulverized fuel and air mixture discharging from the fuel delivery pipe passes into the furnace and a second flow passageway in the annular space between the inner and outer shells through which the secondary air discharging from the secondary air conduit passes into the furnace. Typically, one or more splitter plates are disposed within the inner shell parallel to the axis of the nozzle tip to divide the flow passageway within the inner shell into multiple subpassages. The nozzle tip may be tilted upward or downward in order to direct the fuel-air mixture, discharging into the furnace from the fuel delivery pipe upwardly or downwardly as a means controlling the temperature of the superheated steam produced in heat exchange surface typically disposed at the outlet of the surface in the manner taught by U.S. Pat. No. 2,363,875.
During normal operation of a tangentially fired furnace, a flame is established at one corner which in turn supplies the required ignition energy to stabilize the flame emanating from the corner downstream of and alterally adjacent to it. When load is reduced, the flames emanating from each corner become shorter and, as a consequence, a reduction in the amount of ignition energy available to the downstream corner occurs. As a result, auxiliary fuel such as oil or natural gas must be introduced in each corner adjacent to the pulverized coal-air stream to provide additional ignition energy thereby insuring that a flameout and resultant unit trip will not occur.
Another problem associated with operating a coal-fired burner at low loads results in the fact that the pulverizing mills typically operate with a relatively constant air flow over all load ranges. When furnace load is reduce, the amount of coal pulverized in the mills decreased proportionally while the amount of primary air used to convey the pulverized coal from the mills through the admission assemblies into the furnace remains fairly constant, thereby causing the fuel-air ratio to decrease. When the load on the furnace is reduced to the low levels desired during minimum demand periods, the fuel-air ratio has decreased to the point where the pulverized coal-primary air mixture has become too fuel lean for ignition to stabilize without significant supplemental ignition energy being made available.
One way in which the need for auxiliary fuel firing during low load operation on coal-fired boilers can be reduced is presented in U.S. Pat. No. 4,252,069. This patent disclosed an improved fuel-air admission assembly incorporating a split coal bucket which permits a pulverized coal-fired furnace to be operated at low loads without use of auxiliary fuel to provide stabilization. As disclosed therein, the split coal bucket comprises independent upper and lower coal nozzles pivotally mounted to the coal delivery pipe, the upper and lower coal nozzles being independently tiltable. When the furnace is operating at low loads such as during the minimum demand periods, the primary air and pulverized coal stream discharing from the coal delivery pipe is split into an upper and a lower coal-air stream and independently directed into the furnace by tilting the upper coal nozzle upward and the lower coal nozzle downward. In doing so, an ignition stabilizing pocket is established in the locally low pressure zone created in the void between the spread apart coal-air streams. Hot combustion products are drawn, i.e., recirculated into this low pressure zone, thus providing enough additional ignition energy to the incoming fuel to stabilize the flame.
An additional nozzle tip designed to improve ignition stability, albeit directed to the ignition of low volatile coal rather than ignition at low load operation, is presented in U.S. Pat. No. 2,608,168. Disclosed therein is a coal bucket pivotally mounted to the coal delivery pipe with the flow passageway defined within the inner shell bifurcated into two parallel but spaced apart flow subpassages. Secondary air is discharged into the furnace from the secondary air conduit surrounding the coal delivery pipe through the flow passageway between the inner and outer shells and through the central channel formed between the parallel but spaced apart subpassages formed within the inner shell. Ignition is said to be improved by increasing the contact area between the coal-air mixture discharged from the spaced flow passages of the inner shell and the bounding secondary air streams.
Despite the aforementioned nozzle tip designs, there still exists a need for a nozzle tip of a relatively simple design which inherently provides improved ignition stability at low load operation. There also exists a need for such a nozzle tip which is readily manufactured by fabrication and/or casting.